The present invention relates to a pressure control valve for holding the inside of a pressure circuit at a predetermined set pressure.
A pressure control valve for holding the inside of a pressure circuit at a predetermined set pressure, such as a direct acting relief valve 2 shown in FIG. 21 has been known, for example.
The direct acting type relief valve 2 closes a relief port 3 in communication with a pressure circuit by a poppet valve 6 biased by a pressure adjusting spring 4. When a pressure in the pressure circuit becomes higher than or equal to a predetermined set pressure, the poppet valve 6 is lifted up to communicate the relief port 3 with a drain port 5 so that a part of a fluid in the pressure circuit may flows out through the drain port 5. Therefore, the pressure circuit can be held at the predetermined set pressure. It should be noted that adjustment of the set pressure is performed by varying a biasing force of the spring 4 by turning an adjusting screw 8.
Here, in the direct acting type relief valve 2, the pressure adjusting spring 4 also serves for urging of the poppet valve 6. Therefore, the spring 4 has to be strengthened for use in a pressure circuit of high pressure. However, for this, the valve per se becomes large to limit the strengthening of the spring 4. Therefore, normally, in the circuit of high pressure, a pilot action type relief valve has been used.
As shown in FIG. 22, a pilot actuation type relief valve 10 has such a construction that a relief port 3 is communicated with an upper chamber 14a of a main valve and a direct acting relief valve 18 forming a pilot portion via an orifice 12. When pressure higher than or equal to a predetermined set pressure develops in the pressure circuit, the poppet valve 18a overcomes the biasing force of the spring 18b to move downwardly to open the direct acting relief valve 18. By this, the upper chamber 14a and the drain port 5 are communicated to lower the pressure to open the main valve 14 to flow out the fluid in the pressure circuit toward the drain port 5. The pressure circuit at high pressure can be maintained at the predetermined set pressure.
A pilot portion, in place of the direct acting relief valve 18 of the pilot actuation type relieve valve 10 shown in FIG. 22, including a proportional electromagnetic type pressure control portion for controlling pressure of an upper chamber, such as chamber 14a, by inputting of an electrical pilot signal is provided. The pressure circuit can be controlled automatically at a predetermined range of set pressure.
However, when a proportional electromagnetic type pressure control portion is used, the pressure control valve may have complicate internal structure and a high production cost. Furthermore, since electricity is used, another problem may be encountered in safety, such as the possibility of fire.
The present invention has been worked out in view of the problems set forth above. It is an object to provide a pressure control valve which employs a pilot operation type structure with simple construction not using an electrical signal for automatically controlling a set pressure of a pressure circuit in a step-less manner.
In order to solve the above-mentioned problem, a pressure control valve defined in claim 1 includes a valve body opposing a seat surface of a relief port connected to a pressure circuit, a spring biasing the valve body onto the seat surface, for flowing out a part of fluid within the pressure circuit to hold the fluid pressure in the pressure circuit at a predetermined set pressure when the fluid pressure in the pressure circuit is higher than or equal to the set pressure, a pilot piston is arranged movably in expansion and contraction direction of the spring at an end portion of the spring remote from the valve body, the pilot piston moves in a direction for compressing the spring when a predetermined pressure of a pilot fluid is input, and a spring force of the spring is increased and decreased by varying compression force of the spring by moving the pilot piston depending upon increasing and decreasing of the pressure of the pilot fluid, and a spring initial valve adjusting means which comprises an initial valve adjusting spring for modifying an initial spring force of the spring by varying the pilot piston to a position proximal to the valve body and a position remote from the valve body when the pilot fluid is not input.
By the pressure control valve defined in claim 1, when the pilot fluid does not flow in, the spring urges the valve body onto the seat surface with a predetermined spring force, a pressure in the pressure circuit is maintained at a predetermined set pressure. When the pressure in the pressure circuit becomes higher than or equal to the set pressure, the valve body is opened against the spring force of the spring to flow the fluid of the pressure circuit to the relief port and flow externally to lower pressure than the pressure in the pressure circuit. When the pressure in the pressure circuit becomes the set pressure, the valve body can abut the seat surface of the relief port by the spring force of the spring to maintain the pressure in the pressure circuit at the set pressure.
On the other hand, when the pilot fluid flows into the valve body, the pilot fluid compresses the spring by moving the pilot piston toward the valve body side and can increase the spring force by shortening the spring length to modify the pressure control valve with an increased set pressure. Thus, the pressure control valve of the present invention can freely modify the set pressure, and the fluid pressure in the pressure circuit can be automatically controlled in step-less manner.
Since fluid is used as the pilot signal, in comparison with the conventional proportioning electromagnetic type pressure control valve using the electrical pilot signal, the pressure control valve of the present invention provides satisfactory measure in view of safety, such as fire prevention or the like and lowers production cost with simple structure.
By operation of the spring initial valve adjusting means, the initial spring force of the spring is varied to enable modification of the set pressure within the wide range.
The present invention defined in claim 2, in the pressure control valve as set forth in claim 1, further comprises a spring force upper limit setting means for holding a large spring force of the spring constant by restricting movement of the pilot piston toward the valve body when a pressure of the pilot fluid is elevated up to a predetermined high value.
By the pressure control valve defined in claim 2, when the pressure of the pilot fluid is elevated to high value, a constant large spring force of the spring becomes a force for biasing the valve body onto the seat surface to have the pressure control valve with the upper limit of the set pressure.
The present invention defined in claims 3, in the pressure control valve as set forth in claim 1 or 2, comprises the spring being set with a spring force resisting against a force for moving the pilot piston toward valve body side until the pressure of the pilot fluid is elevated to a predetermined value.
By the pressure control valve defined in claim 3, until the pressure of the pilot fluid is elevated up to the predetermined value, a constant small spring force of the spring becomes a force biasing the valve body onto the seat surface to provide the pressure control valve with the lower limit of the set pressure.
Furthermore, the invention defined in claim 4, in the pressure control valve as set forth in claim 1 or 2, further comprises spring initial valve adjusting means for modifying an initial spring force of the spring by varying the pilot piston to a position proximal to the valve body and a position remote from the valve body when the pilot fluid is not input.
The pressure control valve as set forth in claim 3, further comprises spring initial valve adjusting means for modifying an initial spring force of the spring by varying the pilot piston to a position proximal to the valve body and a position remote from the valve body when the pilot fluid is not input.
By the pressure control valve defined in claim 4, by operation of the spring initial value adjusting means, the initial spring force of the spring is varied to enable modification of the set pressure within a wide range.
It should be noted that the pressure control valve defined in claim 6, a valve body opposing a seat surface of a relief port connected to a pressure circuit and a spring biasing the valve body onto the seat surface. When a fluid pressure in the pressure circuit is higher than or equal to a set pressure, fluid flows out for holding a predetermined set pressure within the pressure circuit. The invention further comprises a bias spring biasing the valve body for opening and closing the relief port to closing side, a pilot piston for biasing the biasing spring to closing side by a pressure of a pilot fluid, an initial valve adjusting spring for biasing the pilot piston to closing side, and a return spring for biasing the valve body or pilot piston to opening side.
The pressure control valve automatically controls the working fluid pressure of the hydraulic circuit by varying the relief pressure in step-less manner by inputting the pilot pressure to the pilot port. Within a range where the pilot pressure input is lower than or equal to the predetermined pressure, the control pressure can be maintained constant. Initial pressure can also be adjusted.